Alloy containing rare earth element, production method thereof, magnetostrictive device, and magnetic refrigerant material

ABSTRACT

A method for producing an RE-containing alloy represented by formula R(T 1−x A x )13 −y  (wherein R represents Ce, etc.; T represents Fe, etc.; and A represents Al, etc; 0.05≦x≦0.2; and −1≦y≦1) including a melting step of melting alloy raw materials at 1,200 to 1,800° C.; and a solidification step of rapidly quenching the molten metal produced through the above step, to thereby form the first RE-containing alloy, wherein the solidification step is performed at a cooling rate of 10 2  to 10 4 ° C./second, as measured at least within a range of the temperature of the molten metal to 900° C.; and an RE-containing alloy, which is represented by a compositional formula of R r T t A a (wherein R and A represent the same meaning as above, T represents Fe, etc.; 5.0 at. %≦r≦6.8 at. %, 73.8 at. %≦t≦88.7 at. %, and 4.6 at. %≦a≦19.4 at. %) and has an alloy microstructure containing an NaZn 13- type crystal structure in an amount of at least 85 mass % and α-Fe in an amount of 5-15 mass % inclusive.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit pursuant to 35 U.S.C. §119(e)(1) ofU.S. Provisional Applications, No. 60/424,015 filed Nov. 6, 2002 and No.60/488,095 filed Jul. 18, 2003.

TECHNICAL FIELD

The present invention relates to a method for producing an alloycontaining a rare earth element (hereinafter referred to asRE-containing alloy), to an RE-containing alloy, to a method forproducing an RE-containing alloy powder, to an RE-containing alloypowder, to a method for producing a sintered RE-containing alloy, to asintered RE-containing alloy, to a magnetostrictive device, and to amagnetic refrigerant. More particularly, the invention relates to atechnique for producing an NaZn₁₃-type RE-containing alloy which issuitable for producing a magnetostrictive device, a magneticrefrigerant, etc.

BACKGROUND ART

Magnetostrictive devices, which generate strain through application of amagnetic field, are employed as magnetostrictive sensors,magnetostrictive vibrators, and similar devices for generating ordetecting displacement with precision. Conventionally, RE-containingintermetallic compounds such as TbFe₂, DyFe₂, and SmFe₂ are employed asmagnetostrictive materials. However, these intermetallic compoundsgenerate only minute displacement, and precise control of the generateddisplacement is difficult. Therefore, these compounds cannot be appliedto magneto-displacement devices for controlling minute displacement.

Meanwhile, GGG (gallium gadolinium garnet) is known to be a magneticrefrigerant which may be applicable to magnetic refrigerators or similardevices. However, GGG has not yet been used in a commercial product,because of its poor refrigeration efficiency upon application of a weakmagnetic field provided by a permanent magnet.

In recent years, an RE-containing alloy having an NaZn₁₃ phase(hereinafter referred to as “NaZn₁₃-type RE-containing alloy”) has beenfound to exhibit large magnetostrain and high magnetocaloric effect. Byvirtue of these properties, this type of alloy is thought to be acandidate magnetostrictive material, magnetic refrigerant, etc.

Specifically, La(Fe_(a)Si_(1−a))₁₃ (0.84≦a≦0.88) is disclosed to exhibita magnetostrain as large as about 0.4% at 200 K under ≧4T (see, forexample, non-patent reference 1).

It is also disclosed that, when the above compound is transformed intoLa(Fe_(a)Si_(1−a))₁₃H_(b) (0.84≦a≦0.88, 1.0≦b≦1.6) through hydrogenabsorption or similar treatment, Curie temperature can be controlled andmagnetocaloric effect can be maintained at a high level (see, forexample, non-patent reference 2).

Conventionally, NaZn₁₃-type RE-containing alloys such asLa(Fe_(a)Si_(1−a))₁₃ (0.84≦a≦0.88) have been produced by weighing alloyraw materials such as high-purity La, Fe, Si, etc., so as to attain adesired alloy composition and mixing; melting the mixture through arcmelting; and heating the product for a considerably long period of time(e.g., at 1,050° C. for 1,000 hours) in order to remove an undesiredphase (see, for example, non-patent reference 2).

In the conventional method for producing NaZn₁₃-type RE-containingalloys, the long-term heat treatment step for removing an undesiredphase lowers productivity and increases costs in the production ofNaZn₁₃-type RE-containing alloys, devices employing the alloys, andother products employing the alloys.

Among RE-containing alloys having an NaZn₁₃ structure, an La—Fe—Si alloyhas been found to exhibit magnetic phase transition concomitant with alarge entropy change in accordance with a change in the externalmagnetic field and to have no temperature hysteresis in themagnetocaloric effect. By virtue of these properties, this type of alloyis considered a candidate magnetic refrigerant.

The magnetic phase transition temperature of the La—Fe—Si alloy can becontrolled by absorbing hydrogen into the alloy, and the change inentropy does not decrease even when hydrogen is absorbed (see non-patentreference 2). Therefore, when the magnetic phase transition temperatureof the alloy is controlled to approximately room temperature and apermanent magnet is employed to generate a magnetic field, the alloy canbe used as a magnetic refrigerant which can work at about roomtemperature.

In addition, this type of alloy, which exhibits a large, isotropicvolume change under application of an external magnetic field, is alsoconsidered a candidate magnetostrictive material (see non-patentreference 1).

A conventionally known method for producing an La—Fe—Si alloy having anNaZn₁₃ structure includes arc-melting raw material metals (i.e., La, Fe,and Si), to thereby form an alloy ingot; heating the alloy ingot in aninert atmosphere at 1,000 to 1,200° C. for 240 hours to 1,000 hours, tothereby form a mother alloy; re-melting the mother alloy; atomizing theformed molten alloy in an atmosphere for cooling, to thereby producespherical particles; and absorbing hydrogen into the particles, tothereby control the magnetic phase transition temperature to apredetermined level (see patent reference 1).

However, the aforementioned conventional method for producing anRE-containing alloy powder has a drawback. Namely, since the methodincludes a long-term heat treatment and two melting steps, productioncosts increase and oxygen content of the alloy increases, even thoughlow-cost material is used.

[Non-Patent Reference 1]

Maya FUJITA and Kazuaki FUKAMICHI, Itinerant-Electron MetamagneticLa(Fe_(x)Si_(1−x))₁₃ Compounds, “Solid-State Physics,” Vol. 37, No. 6,(2002), p. 419-427

[Non-Patent Reference 2]

Maya FUJITA, Shun FUJIEDA, and Kazuaki FUKAMICHI, Large Magnetic Volumeand Magnetocaloric Effect of Itinerant-Electron MetamagneticLa(Fe_(x)Si_(1−x))₁₃ Compounds, “Materia,” Vol. 41, No. 4, (2002), p.269-275

[Patent Reference 1]

Specification of Japanese Patent Application Laid-Open (kokai) No.2003-96547

[Problems to be Solved by the Invention]

The present invention has been conceived under such circumstances. Thus,an object of the present invention is to provide a technique capable ofproducing an NaZn₁₃-type RE-containing alloy at high efficiency withoutperforming a long-period heat treatment step. Another object of theinvention is to provide an NaZn₁₃-type RE-containing alloy producedthrough the technique. Still another object of the invention is toprovide a magnetostrictive device fabricated from the NaZn₁₃-typeRE-containing alloy. Yet another object of the invention is to provide amagnetic refrigerant produced from the NaZn₁₃-type RE-containing alloy.

In addition, an object of the present invention is to provide anRE-containing alloy powder which is easily pulverizable, is not toobrittle, and can be produced at low cost, within a short time, andwithout increasing the oxygen content of the RE-containing alloy powderor a sintered product of the alloy which is employed as a magneticrefrigerant or a magnetostrictive material.

The present inventor has carried out extensive studies in order to solvethe aforementioned problem, and has discovered the following method forproducing an RE-containing alloy, an RE-containing alloy, a method forproducing an RE-containing alloy powder, an RE-containing alloy powder,a method for producing a sintered RE-containing alloy, a sinteredRE-containing alloy, a magnetostrictive device, and a magneticrefrigerant.

The present invention comprises the following items (1) to (22).

(1) A first method for producing a first RE-containing alloy representedby formula R(T_(1−x)A_(x))_(13−y) (wherein R represents at least onespecies selected from among La, Ce, Pr, Nd, Sm, Eu, Th, Dy, Ho, Tm, Yb,Gd, and Lu; T represents at least one species selected from among Fe,Co, Ni, Mn, Pt, and Pd; and A represents at least one species selectedfrom among Al, As, Si, Ga, Ge, Mn, Sn, and Sb (0.05≦x≦0.2; and −1≦y≦1))comprising a melting step of melting alloy raw materials at 1,200 to1,800° C.; and a solidification step of rapidly quenching the moltenmetal produced through the above step, to thereby form the firstRE-containing alloy, wherein the solidification step is performed at acooling rate of 10² to 10⁴° C./second, as measured at least within arange of the temperature of the molten metal to 900° C.

(2) The method for producing an RE-containing alloy according to (1),wherein, in the melting step, the alloy raw material is melted in aninert gas atmosphere at 0.1 to 0.2 MPa.

(3) A method for producing the first RE-containing alloy according to(1), wherein in the solidification step, the molten metal israpid-quenched through any of strip casting, new centrifugal casting,and centrifugal casting.

(4) A method for producing the first RE-containing alloy according to(3), wherein the molten metal is rapidly quenched through strip castingin the solidification step, to obtain strips having a thickness of 0.1to 2.0 mm.

(5) A second method for producing a second RE-containing alloycomprising a melting step and a solidification step for producing afirst RE-containing alloy according to (1), and a heat treatment step ofheating at 900 to 1,200° C. the first RE-containing alloy that isproduced through the solidification step, to thereby form an NaZn₁₃phase.

(6) The second method for producing a second RE-containing alloyaccording to (5), wherein the NaZn₁₃ phase is formed through the heattreatment step, which is performed for a period of from one minute to200 hours.

(7) The second method for producing a second RE-containing alloyaccording to (6), wherein the heat treatment is performed at atemperature of 1080° C. to 1200° C. and for a period of from 3 to 42hours.

(8) The first RE-containing alloy which is obtainable through the methodof any one of (1) to (4).

Through the first methods of the present invention for producing a firstRE-containing alloy (that is the above (1) to (4)), an RE-containingalloy suitably used for producing an NaZn₁₃-type RE-containing alloys(i.e., a starting alloy for an NaZn₁₃-type RE-containing alloy) areproduced. NaZn₁₃-type RE-containing alloys are produced through thesecond methods of the present invention for producing RE-containingalloys ((5) and (6)).

(9) A first RE-containing alloy, which is represented by formulaR(T_(1−x)A_(x))_(13−y) (wherein R represents at least one speciesselected from among La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Tm, Yb, Gd, andLu; T represents at least one species selected from among Fe, Co, Ni,Mn, Pt, and Pd; and A represents at least one species selected fromamong Al, As, Si, Ga, Ge, Mn, Sn, and Sb (0.05≦x≦0.2; and −1≦y≦1)), andwhich comprises an R-rich phase, having a relatively high rare earthmetal (R) content, and an R-poor phase, having a relatively low rareearth metal (R) content, wherein the R-rich phase and the R-poor phaseare dispersed at a phase spacing of 0.01 to 100 μm.

In the present specification, the R-rich phase spacing, the R-poor phasespacing, and the size of each phase are evaluated by use ofback-scattered electron images of the alloy observed under a scanningelectron microscope.

In a back-scattered electron image of the alloy, a portion having alarge average atomic weight is observed as a white image, whereas aportion having a small average atomic weight is observed as a blackimage. In other words, the R-rich phase is observed as a white image,and the R-poor phase, having a rare earth metal content lower than thatof the R-rich phase, is observed as a gray image. In a specificprocedure, a back-scattered electron image of an alloy sample is takenat an appropriate magnification as a rectangular image, and the imagedata are converted to two values corresponding to black and white by useof image processing software. Subsequently, a lateral segment and alongitudinal segment through the center, and two diagonals (total foursegments) are drawn in the rectangular image. The total lengths of thewhite portions that intersect each segment are measured, and the fourmeasured lengths are averaged, to thereby derive the size of the R-richphase (i.e., equivalent to R-poor phase spacing). In a similar manner,the total length of the black portions that intersect each segment ismeasured, and the four measured lengths are averaged, to thereby derivethe size of the R-poor phase (i.e., equivalent to R-rich phase spacing).

(10) A second RE-containing alloy, which is represented by formulaR(T_(1−x)A_(x))_(13−y) (wherein R represents at least one speciesselected from among La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Tm, Yb, Gd, andLu; T represents at least one species selected from among Fe, Co, Ni,Mn, Pt, and Pd; and A represents at least one species selected fromamong Al, As, Si, Ga, Ge, Mn, Sn, and Sb (0.05≦x≦0.2; and −1≦y≦1)),wherein the alloy has an NaZn₁₃ phase content of at least 90 vol. %.

(11) A magnetostrictive device provided from the second RE-containingalloy according to (10).

(12) A magnetic refrigerant provided from the second RE-containing alloyaccording to (10).

The magnetostrictive device and magnetic refrigerant of the presentinvention are characterized by being produced from the aforementionedsecond RE-containing alloy of the present invention (an NaZn₁₃-typeRE-containing alloy).

(13) An RE-containing alloy, which is represented by a compositionalformula of R_(r)T_(t)A_(a) (wherein R represents at least one rare earthelement selected from among La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Tm, Yb,Gd, and Lu; T collectively represents transition metal elementscontaining at least Fe atoms, a portion of the Fe atoms being optionallysubstituted by at least one species selected from among Co, Ni, Mn, Pt,and Pd; A represents at least one element selected from among Al, As,Si, Ga, Ge, Mn, Sn, and Sb; and r, t, and a have the followingrelationships: 5.0 at. %≦r≦6.8 at. %, 73.8 at. %≦t≦88.7 at. %, and 4.6at. %≦a≦19.4 at. %) and having an alloy microstructure containing anNanZn₁₃-type crystal structure in an amount of at least 85 mass % andα-Fe in an amount of 5-15 mass % inclusive.

(14) A method for producing an RE-containing alloy powder, comprisingpulverizing, by mechanical means, the RE-containing alloy according to(13) to a powder having a mean particle size of 0.1 μm to 1.0 mm.

(15) An RE-containing alloy powder comprising an RE-containing alloyaccording to (13), which has a mean particle size of 0.1 μm to 1.0 mm.

(16) A magnetic refrigerant comprising the sintered RE-containing alloypowder according to (15), wherein the Curie temperature of the magneticrefrigerant has been controlled through absorption of hydrogen in thesintered RE-containing alloy.

(17) A method for producing a sintered RE-containing alloy, whichcomprises compacting an RE-containing alloy powder produced through amethod for producing an RE-containing alloy powder as described in (14),and sintering the compact.

(18) The method for producing a sintered RE-containing alloy describedin (17), wherein the sintering is performed at 1,200° C. to 1,400° C.

(19) The method for producing a sintered RE-containing alloy describedin (17) or (18), wherein, after completion of sintering theRE-containing alloy powder, the sintered alloy is maintained in ahydrogen atmosphere at 200° C. to 300° C., to thereby absorb hydrogeninto the sintered alloy.

(20) A sintered RE-containing alloy, which is formed by compacting theRE-containing alloy powder as recited in (15), and sintering thecompact.

(21) A magnetostrictive material comprising the sintered RE-containingalloy as recited in (20), wherein the Curie temperature of themagnetostrictive material has been controlled through absorption ofhydrogen into the sintered RE-containing alloy.

(22) A magnetic refrigerant comprising the sintered RE-containing alloyas recited in (20), wherein the Curie temperature of the magneticrefrigerant has been controlled through absorption of hydrogen into thesintered RE-containing alloy.

According to the present invention, there can be provided a techniquecapable of producing an NaZn₁₃-type RE-containing alloy at highefficiency without performing long-term heat treatment; an NaZn₁₃-typeRE-containing alloy produced through the technique; and amagnetostrictive device and a magnetic refrigerant obtained from theNaZn₁₃-type RE-containing alloy.

Further, according to the present invention, a magnetostrictive materialand a magnetic refrigerant formed of an RE-containing alloy (e.g.,La—Fe—Si) having an NaZn₁₃ structure can be reliably produced at lowcost, compared with conventional methods. The present inventioncontributes toward mass production of magnetic refrigerator andmagnetostrictive devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical production apparatus suitably used in the firstmethod of the present invention for producing an RE-containing alloy.

FIG. 2 shows a back-scattered electron image of a cross-section of asample of La(Fe_(0.89)Si_(0.11))₁₃ alloy produced through strip casting.

FIG. 3 is an X-ray diffraction chart of an alloy powder which has beenproduced through rapid quenching based on strip casting, followed byheat treatment at 1,100° C. for 12 hours.

FIG. 4 shows a back-scattered electron image of a cross-section of analloy sample which has been treated at 1,100° C. for 12 hours.

FIG. 5 is a graph showing the relationship between the amount ofLa(Fe_(0.89)Si_(0.11))₁₃ formed and the time for heat-treating at 1,100°C.

FIG. 6 shows a back-scattered electron image of a cross-section of analloy sample which has been treated at 1,100° C. for 200 hours.

FIG. 7 is a graph showing the relationship between the sinteringtemperature and the sintered product density.

FIG. 8 is a graph showing the relationship between the temperature forhydrogen-treating of sintered La(Fe_(0.89)Si_(0.11))₁₃ (absorptiontemperature) and the lattice constant of the sintered product.

FIG. 9 is a graph showing the relationship between the dehydrogenationtemperature and the lattice constant of La(Fe_(0.89)Si_(0.11))₁₃.

FIG. 10 is a back-scattered electron image of an RE-containing alloysample produced in the Example of the present invention.

FIG. 11 is a back-scattered electron image of an RE-containing alloysample produced in the Example of the present invention.

FIG. 12 is a back-scattered electron image of an RE-containing alloysample produced in the Comparative Example of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[First Method for Producing a First RE-Containing Alloy]

The first method of the present invention for producing a firstRE-containing alloy is directed to a method for producing a startingalloy (the first RE-containing alloy) for an NaZn₁₃-type RE-containingalloy that is represented by formula R(T_(1−x)A_(x))_(13−y) (wherein Rrepresents at least one rare earth metal selected from among La, Ce, Pr,Nd, Sm, Eu, Tb, Dy, Ho, Tm, Yb, Gd, and Lu; T represents at least onetransition metal selected from among Fe, Co, Ni, Mn, Pt, and Pd; and Arepresents at least one element selected from among Al, As, Si, Ga, Ge,Mn, Sn, and Sb (0.05≦x≦0.2; and −1≦y≦1)).

The method generally includes a melting step (1) of melting an alloy rawmaterial; and a solidification step (2) of solidifying the molten metalproduced through the above step, to thereby form an RE-containing alloy(the first RE-containing alloy), wherein the solidification is performedthrough the “rapid quenching method.”

Each step will next be described in detail.

<Melting Step>

In the melting step, alloy raw materials; i.e., materials containing arare earth metal (R), a transition metal (T), and an element (A),respectively, are mixed, and the mixture is melted.

No particular limitation is imposed on the material containing a rareearth element (R), so long as the material predominantly contains atleast one species selected from among La, Ce, Pr, Nd, Sm, Eu, Th, Dy,Ho, Tm, Yb, Gd, and Lu. Examples of such materials which can be employedinclude rare earth metals (purity: ≧90 mass %, the balance beingunavoidable impurities such as Al, Fe, Mo, W, C, O, or N) and mischmetals predominantly containing La, Ce, etc. (rare earth metal content:≧90 mass %, the balance being unavoidable impurities such as Al, Fe, Mo,W, C, O, or N).

No particular limitation is imposed on the material containing atransition metal (T), so long as the material predominantly contains atleast one species selected from among Fe, Co, Ni, Mn, Pt, and Pd.Examples of such materials which can be employed include pure metalssuch as Fe, Co, and Ni (purity: ≧99 mass %).

No particular limitation is imposed on the material containing anelement (A), so long as the material predominantly contains at least onespecies selected from among Al, As, Si, Ga, Ge, Mn, Sn, and Sb. Examplesof such materials which can be employed include metallic silicon(purity: ≧95 mass %, the balance being unavoidable impurities such asPb, As, Fe, Cu, Bi, Ni, C, 0, or N), metallic Ga, and pure Al.

These materials containing a rare earth metal (R), a transition metal(T), and an element (A), respectively, are weighed so as to attain analloy composition represented by formula R(T_(1−x)A_(x))_(13−y)(0.05≦x≦0.2; and −1≦y≦1), followed by mixing.

Specifically, when an alloy of La(Fe_(0.88)Si_(0.12))₁₃ is to beproduced, the compositional proportions in terms of La, Fe, and Sicontained in the raw materials preferably fall within the ranges of 16.8to 17.3 mass %, 78.3 to 80.1 mass %, and 4.8 to 5.0 mass %,respectively.

The thus-produced alloy material mixture is melted by heating at 1,200to 1,800° C. The melting is preferably performed in an inert gasatmosphere at 0.1 MPa (atmospheric pressure) to 0.2 MPa Examples of theinert gas include Ar and He.

A heating temperature lower than 1,200° C. is not preferred because anundesired phase other than the R-rich phase and the R-poor phase may beformed, whereas a heating temperature higher than 1,800° C. is notpreferred, because vaporization of a rare earth metal is promotedexcessively, leading to difficulty in controlling the alloy composition.An inert gas atmosphere pressure lower than 0.1 MPa is not preferred,because vaporization of a rare earth metal is promoted excessively,leading to difficulty in controlling the alloy composition, whereas aninert gas atmosphere pressure higher than 0.2 MPa is not preferred,because the inert gas tends to migrate into the molten metal, therebyyielding an alloy having a large number of pores.

<Solidification Step>

In the solidification step, the molten metal produced through the abovemelting step is rapidly quenched, to thereby form an RE-containingalloy. Rapid quenching may be performed through strip casting, newcentrifugal casting (employing a tundish of a rotatable disk type),centrifugal casting, or a similar method.

According to the present invention, the solidification step is performedat a cooling rate of 10² to 10⁴° C./second, preferably 5×10² to 3×10³°C./second, as measured at least within a range of the temperature of themolten metal to 900° C.

The present inventor has found that a uniform alloy metallographicmicrostructure which includes a crystalline phase can be formed bycontrolling the cooling rate in the above manner. More specifically, theinventor has found that there can be produced an RE-containing alloywhich includes an R-rich phase (relatively high rare earth metal (R)content) and an R-poor phase (relatively low rare earth metal (R)content), with each phase being minute and being dispersed at a smallphase spacing of 0.01 to 100 μm.

The inventor has also found that the R-rich phase and the R-poor phaseare transformed into an NaZn₁₃ phase by heating the RE-containing alloywithin a period of time as short as 200 hours or less, to therebyeffectively produce an NaZn₁₃-type RE-containing alloy. The reason forthe effective production of the alloy is that atoms for forming theNaZn₁₃ phase are effectively diffused, to thereby complete NaZn₁₃ phaseformation within a shorter period than in conventional methods.

When the cooling rate as measured within a range of the temperature ofthe molten metal to 900° C. is less than 10²° C./second, the size andphase spacing of the R-rich phase and the R-poor phase increases,leading to difficulty in formation of a uniform NaZn₁₃ phase throughheat treatment, although an alloy microstructure including the R-richphase and the R-poor phase is formed. When the cooling rate as measuredwithin the same range is more than 10 ⁴° C./second, the formed alloymicrostructure includes an amorphous phase containing a transition metal(T), leading to considerable deterioration in pulverization andprocessing characteristics. These two cases are not preferred.

<Production Apparatus>

With reference to FIG. 1, an exemplary production apparatus suitablyused in the first method of the present invention for producing anRE-containing alloy will be briefly described, taking as an example thecase in which rapid quenching is performed through the strip castingmethod.

FIG. 1 shows the above apparatus including a crucible 1, a tundish 2, acooling roller 3, and a receiving box 4.

In the apparatus, alloy raw materials are melted in the crucible 1,whereby a molten metal 5 is formed.

The formed molten metal 5 is poured via the tundish 2 onto thecylindrical cooling roller 3, which is rotating in a predetermineddirection (counterclockwise in FIG. 1). The cooling roller 3 is formedof a copper roller or the like cooled by water or a similar medium. Themolten metal 5 is rapidly quenched to 900° C. or lower through contactwith the roller, to thereby form an alloy. The rate for cooling themolten metal 5 can be regulated by modifying the rotating speed (asrepresented by peripheral velocity) of the cooling roller 3; modifyingthe amount of the molten metal poured onto the cooling roller 3; ormodifying a similar parameter.

The thus-formed alloy is removed from the cooling roller 3 in the formof strips 6, which are collected into the receiving box 4. According tothe present invention, the thickness of the formed alloy strips 6 ispreferably regulated to 0.1 to 2.0 mm by modifying the amount of themolten metal poured onto the cooling roller 3 or a similar parameter.Regulation of the thickness of the alloy strips 6 within the above rangeprovides an RE-containing alloy which is formed of an R-rich phase andan R-poor phase, the phases having a minute size and being dispersed ata minute phase spacing, and which has an excellent pulverizationcharacteristic.

The thus-collected strips 6 are cooled in the receiving box 4 to roomtemperature, and then removed from the receiving box. In this case, therate of cooling the alloy collected in the receiving box 4 is preferablycontrolled through thermal insulation or forced cooling of the receivingbox 4. Thus, by controlling the rate at which the alloy is cooled toroom temperature performed after the alloy is cooled to 900° C. or lowerby means of the cooling roller 3, uniformity in the alloy microstructurecan be further enhanced.

[Second Method for Producing a Second RE-Containing Alloy]

The second method of the present invention for producing anRE-containing alloy is directed to a method for producing an NaZn₁₃-typeRE-containing alloy from a starting alloy for an NaZn₁₃-typeRE-containing alloy produced through the first method for producing anRE-containing alloy.

The production method includes a heat treatment step (3) of heating thestarting alloy for NaZn₁₃-type RE-containing alloy that is producedthrough the aforementioned first method. More specifically, the heattreatment step includes heating at 900 to 1,200° C. the starting alloyfor NaZn₁₃-type RE-containing alloy that is produced through theaforementioned first method, to thereby form an NaZn₁₃ phase. The heattreatment is preferably performed under reduced pressure or in vacuum.

The present inventor has found that the R-rich phase and the R-poorphase are transformed into an NaZn₁₃ phase by heating the RE-containingalloy at 900 to 1,200° C. for a short period of time falling within arange of one minute to 200 hours, to thereby produce an NaZn₁₃-typeRE-containing alloy at remarkably high efficiency. The inventor has alsofound that the produced NaZn₁₃-type RE-containing alloy has an NaZn₁₃phase content of at least 90 vol. %.

The ratio of the volume of NaZn₁₃ phase contained in the alloy to thatof non-NaZn₁₃ phase(s) can be determined by identifying each crystalphase through powder x-ray diffractometry; calculating the ratio of“area of the NaZn₁₃ phase” to “area exhibiting contrast differing fromthat of the NaZn₁₃ phase,” on the basis of a back-scattered electronimage observed under a scanning electron microscope; and converting thearea ratio to the corresponding volume ratio.

When the heat treatment temperature is lower than 900° C., a uniformNaZn₁₃ phase cannot be formed even when the heat treatment is performedfor 200 hours or longer. When the temperature is higher than 1,200° C.,the NaZn₁₃ phase may be separated to form an undesired phase. Both casesare not preferred.

As described in detail hereinabove, the method according to the presentinvention includes rapid quenching, under predetermined conditions, ofthe molten metal produced by melting raw materials. Therefore, themethod provides a starting alloy for an NaZn₁₃-type RE-containing alloyhaving an alloy microstructure in which an R-rich phase and an R-poorphase are minutely and uniformly dispersed and being suitable forproducing an NaZn₁₃-type RE-containing alloy. From the starting alloy,an NaZn₁₃-type RE-containing alloy can be produced through heattreatment for a short period of time of 200 hours or less.

Thus, according to the present invention, productivity in the productionof an NaZn₁₃-type RE-containing alloy and devices and other productsfabricated by use of the alloy can be remarkably enhanced, andproduction costs can be remarkably reduced.

In addition, the starting alloy in which the R-rich phase and the R-poorphase are minutely and uniformly dispersed also has an excellentpulverization characteristic. Therefore, the starting alloy hasexcellent processability and is readily formed into a desired shapethrough pulverization, compacting, and sintering. Thus, an NaZn₁₃-typeRE-containing alloy product having a desired shape can be readilyproduced from the corresponding starting alloy of the same shape.

According to the present invention, a high-quality NaZn₁₃-typeRE-containing alloy having an NaZn₁₃ phase content of 90 vol. % orhigher can be provided. Thus, from the NaZn₁₃-type RE-containing alloy,high-performance magnetostrictive devices and magnetic refrigerants canbe provided.

[A Third RE-Containing Alloy, an Alloy Powder Comprising the ThirdRE-Containing Alloy, and a Method for Producing the Alloy Powder]

The present invention is also directed to a third RE-containing alloycomprising a rare earth element R (wherein R represents at least onespecies selected from among La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Tm, Yb,Gd, and Lu); a transition metal element T (wherein T collectivelyrepresents transition metal elements containing at least Fe atoms, aportion of the Fe atoms being optionally substituted by at least onespecies selected from among Co, Ni, Mn, Pt, and Pd); and other elementsA (wherein A represents at least one species selected from among Al, As,Si, Ga, Ge, Mn, Sn, and Sb); having a composition (atomic percentage ofR, T, and A represented by r, t, and a, which have the followingrelationships: 5.0 at. %≦r≦6.8 at. %, 73.8 at. %≦t≦88.7 at. %, and 4.6at. %≦a≦19.4 at. %), and having an alloy microstructure containing anNaZn₁₃-type crystal structure in an amount of at least 85 mass %. Theinvention is also directed to an alloy powder comprising theRE-containing alloy, and a method for producing the alloy powder.

In the third RE-containing alloy according to the present invention, theaforementioned rare earth elements R, transition metal elements T, andother elements A are essential for producing an alloy having anNaZn₁₃-type crystal structure.

Conventionally, compositional proportions (rare earth element R,transition metal element T, and other element(s) A) suitable forproducing the above alloy are considered to be the following: 5.5 at.%≦r≦7.1 at. %, 73.8 at. %≦t≦88.7 at. %, and 4.6 at. %≦a≦19.4 at. %.Theoretical compositional proportions of r, t, and a in theaforementioned formula for producing the alloy having an NaZn₁₃-typecrystal structure are represented by r: (t+a)=1:13; i.e., r is 7.1 at.%.

However, according to the present invention, the proportion of the rareearth element R contained in the alloy is regulated to 5.0 at. %≦r≦6.8at. %, which is less than the theoretical range. As a result, α-Feremains in the RE-containing alloy in an amount of 5 mass % or more,whereby mechanical strength of the sintered RE-containing alloy whichhas undergone hydrogen absorption can be maintained.

Notably, the third RE-containing alloy according to the presentinvention in some cases contains impurities such as O, C, and N. In sucha case, the amount of each impurity; i.e., O, C, or N, is preferably 1mass % or less and as small as possible.

The third RE-containing alloy according to the present invention ispreferably cast through rapid quenching so as to conveniently form,through heat treatment, an RE-containing alloy having an alloymicrostructure containing an NaZn₁₃-type crystal structure in an amountof at least 85 mass %. Examples of rapid quenching methods include stripcasting (SC) and centrifugal casting. Taking the SC method as anexample, casting of the RE-containing alloy according to the presentinvention will be described in detail.

According to the method for producing the third RE-containing alloyaccording to the present invention, the alloy raw material is melted inthe crucible 1 at 1,500° C. to 1,800° C. under inert gas such as Ar orHe. The molten alloy 5, prepared by melting the alloy raw material, ispoured via the tundish 2 onto the copper roller 3, which is cooled bywater and is rotating in a direction indicated by the arrow shown inFIG. 1, to thereby rapidly quench the alloy. During this process, thecooling rate, as measured within a range of the temperature of themolten alloy to 900° C., is controlled to 10² to 10⁴° C./second,preferably to 5×10²° C./second to 3×10³° C./second. By controlling thecooling rate in such a manner, a uniform alloy micro-structure having acrystalline phase can be readily formed. The thus-produced alloy mayalso be called “RE-containing alloy which has not yet undergone heattreatment.”

When the cooling rate is less than 10²° C./second, the R-rich phasespacing, the R-poor phase spacing, and the size of each phase increase.In this case, even when a subsequent heat treatment is performed, auniform NaZn₁₃-type crystal structure is difficult to form. When thecooling rate is in excess of 10⁴° C./second, the formed alloy assumes amixture of amorphous metal and Fe, resulting in poor productivity. Bothcases are not preferred. The molten alloy cooling rate can be controlledto a desired value by modifying the rotating speed (as represented byperipheral velocity) of the copper roller 3 or by modifying the amountof the molten alloy poured onto the copper roller 3.

FIG. 2 is a back-scattered electron image of a cross-section of a sampleof La(Fe_(0.89)Si_(0.11))₁₃ alloy, which is an example of an alloyhaving a composition of R_(r)T_(t)A_(a) cast through the above rapidquenching method. In a back-scattered electron image of the alloy, aportion having a large average atomic weight is observed as a whiteimage, whereas a portion having a small average atomic weight isobserved as a black image. Thus, the R-rich phase, having a high rareearth metal content, is observed as a white image, and the R-poor phase,having a rare earth metal content lower than that of the R-rich phase,is observed as a gray image. As shown in FIG. 2, the alloy producedthrough the aforementioned rapid quenching method is formed of an R-richphase, having a relatively high rare earth metal content, and an R-poorphase, having a relatively low rare earth metal content. Theback-scattered electron image of FIG. 2 indicates that the R-rich phase(white image) has a size of 5 μm or less, that the R-poor phase (grayimage) has a diameter of 10 μm or less, and that the phases areuniformly distributed in the microstructure.

In the microstructure of the cast RE-containing alloy, each of theR-rich phase and R-poor phase has a size (diameter) of 30 μm or less,preferably 10 μm or less. Preferably, both phases are uniformlydispersed in the structure. When an RE-containing alloy has such amicrostructure, the diffusion path of elements between the R-rich phaseand the R-poor phase is shortened. Thus, α-Fe can be dispersed minutelyand uniformly through heat treatment carried out for a short period oftime. As a result, an RE-containing alloy containing an NaZn₁₃-typecrystal structure in an amount of 85 mass % or more can be readilyproduced through heat treatment carried out for a short period of time.An RE-containing alloy containing an R-rich phase and an R-poor phaseeach having a size (diameter) of 30 μm or less can be reliably producedby casting through rapid quenching at a cooling rate of 10² to 10⁴°C./second, as measured within a range of the temperature of the moltenalloy to 900° C.

Through heat treatment of an alloy which has been cast through the aboverapid quenching method (i.e., RE-containing alloy which has not yetundergone heat treatment), the amount of the NaZn₁₃ structure containedin the alloy microstructure can be increased to 85 mass % or more. FIG.3 is an X-ray diffraction chart of an alloy powder which has beenproduced by casting the alloy through strip casting, followed by heattreatment at 1,100° C. for 12 hours. Notably, the alloy shown in FIG. 3has been produced from La, Fe, and Si serving as raw materials ofcomponents R, T, and A, respectively. The compositional proportions (at.%) are as follows: r=6.8%; t=82.9%; and a=10.3%.

The amount of NaZn₁₃-structure La(Fe_(0.89)Si_(0.11))₁₃ formed in theRE-containing alloy shown in FIG. 3 can be calculated in a simplemanner. Specifically, the intensity of the maximum peak attributed toLa(Fe_(0.89)Si_(0.11))₁₃ in powder X-ray diffractiometry is determined;the intensities of the maximum peaks attributed to the phases other thanLa(Fe_(0.89)Si_(0.11))₁₃ are determined; and the intensity forLa(Fe_(0.89)Si_(0.11))₁₃ is divided by the sum of the intensity forLa(Fe_(0.89)Si_(0.11))₁₃ and the intensities for the other phases.

For example, as indicated by the arrows shown in FIG. 3, the maximumpeak attributed to La(Fe_(0.89)Si_(0.11))₁₃ is observed at approximately38.4° through measurement by use of a CuKα ray, whereas the maximum peakattributed to α-Fe (a phase other than La(Fe_(0.89)Si_(0.11))₁₃) isobserved at 44.7°. Thus, the amount of formed La(Fe_(0.89)Si_(0.11))₁₃can be calculated from the following formula: 100×(the peak intensity at38.4°)/(the peak intensity at 38.4°+the peak intensity at 44.7°) (%).The calculation requires a calibration curve indicating the relationshipbetween the intensity ratio and the phase ratio. According to the abovecalculation method on the basis of the chart of FIG. 3 and thecalibration curve, the amount of La(Fe_(0.89)Si_(0.11))₁₃ formed wascalculated to be 92 mass % and that of α-Fe formed was calculated to be8 mass %.

These results are consistent with the features of the back-scatteredimage. Specifically, in the back-scattered electron image of FIG. 2showing the microstructure of the RE-containing alloy which has not yetundergone heat treatment, an R-rich phase and an R-poor phase eachhaving a small size are observed. However, the back-scattered electronimage of FIG. 4 showing the microstructure of the RE-containing alloywhich has undergone heat treatment indicates that the structure isformed of La(Fe_(0.89)Si_(0.11))₁₃ and a small amount of α-Fe.

As is clear from FIGS. 2 to 4, in the alloy according to the presentinvention which has been cast through rapid quenching, the R-rich phaseand R-poor phase tend to be transformed through heat treatment toNaZn₁₃-structure La(Fe_(0.89)Si_(0.11))₁₃, and the amount thereof can bereadily elevated to 85 mass % or more.

The temperature for treating the RE-containing alloy which has been castthrough the aforementioned method preferably falls within a range of1,080° C. to 1,200° C. For example, when the RE-containing alloy isheat-treated in vacuum at a temperature increase rate of 10 K/minute anda retention time at a maximum heating temperature of one hour, theR-rich phase and the R-poor phase contained in the alloy are removed,and La(Fe_(0.89)Si_(0.11))₁₃ content increases through heating of thealloy at 1,080° C. to 1,200° C. The La(Fe_(0.89)Si_(0.11))₁₃ contentexceeds 85 mass %.

However, when the alloy is heat-treated at a temperature higher than1,200° C., rare earth metal elements present on the surface of an alloypiece are evaporated, resulting in rare earth component deficiency. Thedeficiency induces decomposition of the NaZn₁₃ phase. In addition, thesize of α-Fe remaining in the alloy increases, thereby affectingpulverization performance of the alloy for producing fine powder.Therefore, a heat treatment temperature higher than 1,200° C. is notpreferred. When the heat treatment temperature is lower than 1,080° C.,the amount of the NaZn₁₃ phase formed fails to reach 85%, and inaddition to α-Fe, La and lamellar-form Fe tend to remain in the alloy,which is not preferred.

FIG. 5 is a graph showing the relationship between the amount ofLa(Fe_(0.89)Si_(0.11))₁₃ formed and the retention time for heat-treatingan RE-containing alloy at a maximum retention temperature of 1,100° C.As is clear from FIG. 5, in the case where the alloy is maintained at1,100° C., the amount of La(Fe_(0.89)Si_(0.11))₁₃ formed graduallydecreases when the heat treatment time exceeds 12 hours. A conceivablereason for the decrease is that a rare earth element component presentin the near surface of the alloy piece is released and evaporated toyield an alloy microstructure as shown in FIG. 6, which should havedesirably yielded a uniform microstructure as shown in FIG. 4. Formationof a defect resulting from the mentioned release or evaporation is notpreferred, since pulverization characteristics are affected. Accordingto the findings from FIGS. 4 and 5, the heat treatment is preferablyperformed at 1,080 to 1,200° C. for 3 to 42 hours. More preferably, theheat treatment is performed at a maximum heat treatment temperature of1,100 to 1,120° C. and a retention time of 6 to 12 hours.

The RE-containing alloy which has been cast through rapid quenching andhas undergone heat treatment (the third RE-containing alloy) assumes theform of flakes, which themselves are not suited for producing a magneticrefrigerant or a magnetostrictive material. Therefore, the flakes arepulverized to form a powder having a mean particle size of 0.1 μm to 1.0mm. The powder itself or a sintered product obtained by compacting andsintering the powder is employed as a magnetic refrigerant or amangetostrictive material. For example, a powder having a particle sizeof at least 200 μm is preferably employed as a magnetic refrigerantwithout further treatment, whereas a powder having a particle size lessthan 200 μm is preferably sintered to provide a magnetostrictive deviceor a magnetic refrigerant.

The aforementioned third RE-containing alloy can be pulverized byvarious mechanical means in accordance with the target mean particlesize of the powder; e.g., a jaw crusher (500 μm or more); a disk mill(50 to 500 μm); and an attriter or ajet mill employing an inert gas suchas nitrogen or argon (50 μm or less). The produced powder is sieved inaccordance with need, to thereby form a powder having a desired particlesize. When a jet mill is employed, the shape of the powder can beregulated by controlling the amount of alloy placed in a container of apulverizer and the pressure of pulverization gas.

When the amount (as determined through powder X-ray diffractometry) ofthe NaZn₁₃ structure formed in the aforementioned third RE-containingalloy is less than 85 mass % and α-Fe content (as determined throughpowder X-ray diffractometry) is more than 15 mass %, ease ofpulverization of the alloy by mechanical means is considerablydeteriorated, whereas when the amount of the NaZn₁₃ structure formed inthe alloy is 85 mass % or more, such an alloy is brittle and readilypulverized. Thus, when the third RE-containing alloy is mechanicallypulverized, the amount of the NaZn₁₃ structure formed in the alloy andthe α-Fe content must be controlled to 85 mass % or more and 15 mass %or less, respectively.

The amount of the NaZn₁₃ structure formed in the third RE-containingalloy and the α-Fe content can be controlled by modifying thecomposition of the alloy and the cast alloy heating conditions. In orderto control the amount of the NaZn₁₃ structure formed in theRE-containing alloy to 85 mass % or more and control the α-Fe content to15 mass % or less, an RE-containing alloy is cast through rapidquenching, followed by further heat treatment at 1,080° C. to 1,200° C.for 3 to 42 hours, the RE-containing alloy being represented by acompositional formula of R_(r)T_(t)A_(a) (wherein R represents at leastone rare earth element selected from among La, Ce, Pr, Nd, Sm, Eu, Th,Dy, Ho, Tm, Yb, Gd, and Lu; T collectively represents transition metalelements containing at least Fe atoms, a portion of the Fe atoms beingoptionally substituted by at least one species selected from among Co,Ni, Mn, Pt, and Pd; A represents at least one element selected fromamong Al, As, Si, Ga, Ge, Mn, Sn, and Sb; and r, t, and a have thefollowing relationships: 5.0 at. %≦r≦6.8 at. %, 73.8 at. %≦t≦88.7 at. %,and 4.6 at. %≦a≦19.4 at. %).

In the case in which the RE-containing alloy powder is used withoutperforming further treatment, the mechanical strength of the alloypowder is enhanced by maintaining the amount of α-Fe contained inparticles of the powder at 5 mass % or more, thereby preventing crushingof the powder. As a result, when the powder is employed as, for example,a magnetic refrigerant, clogging of a filter is prevented, therebyenhancing operational reliability. Therefore, the third RE-containingalloy powder preferably contains α-Fe in an amount of 5-15 mass %inclusive.

In the case in which the third RE-containing alloy powder is compacted,a compact having sufficient mechanical strength is produced at acompacting pressure of 0.8 t/cm² or higher. Such a compact can be usedwithout any problem in a subsequent step, such as conveying. However,when the alloy powder is compacted at a pressure lower than 0.8 t/cm²,the produced compact has poor mechanical strength and is difficult touse, because of chipping.

When the compact is sintered in a vacuum or an inert gas atmosphere at1,200 to 1,400° C., preferably 1,280 to 1,300° C., a high-densityRE-containing alloy sintered product can be produced. FIG. 7 is a graphshowing the relationship between sintering temperature and density ofthe sintered product, the product having been obtained from an alloypowder having a particle size of 50 to 100 μm. As is clear from FIG. 7,sintering at 1,280° C. or more results in sufficient density of thesintered product. For example, a compact produced fromLa(Fe_(0.89)Si_(0.11))₁₃ powder (particle size: 50 to 100 μm) issintered at 1,280° C. for three hours, followed by heating at 1,100° C.for 12 hours, to thereby produce sintered La(Fe_(0.89)Si_(0.11))₁₃having a density of 6.9 g/cm³ or more.

The Curie temperature of the sintered RE-containing alloy can becontrolled by absorbing hydrogen into the sintered alloy. FIG. 8 is agraph showing the relationship between the absorption temperature andthe lattice constant of the sintered RE-containing alloy which hasabsorbed hydrogen. As shown in FIG. 8, no change in lattice constant isobserved from room temperature to 200° C., indicating that hydrogenabsorption does not occur. At 200° C. and higher, an increase in latticeconstant induced by hydrogen absorption is observed. Accordingly, inorder to absorb hydrogen into the sintered alloy, the sintered alloy ispreferably maintained in hydrogen at atmospheric pressure for one houror longer at a maximum temperature of 200 to 300° C., more preferably230 to 270° C., and cooled in the hydrogen atmosphere. Thus, absorbinghydrogen into the sintered alloy changes the lattice constant, therebycontrolling the Curie temperature of the sintered RE-containing alloy.

After completion of hydrogen absorption, excessive hydrogen absorbed inthe sintered alloy can be released by heating the sintered alloy at 100to 200° C. under Ar or in vacuum. FIG. 9 is a graph showing change inlattice constant of a sintered alloy which has undergone excessivehydrogen absorption by heating at 400° C. in hydrogen at atmosphericpressure, followed by dehydrogenation at various temperatures. As isclear from FIG. 9, the lattice constant decreases as the dehydrogenationtemperature is elevated. In particular, decrease in lattice constant isremarkable from about 190° C.

When hydrogen is absorbed into a sintered RE-containing alloy having anα-Fe content less than 5 mass %, numerous cracks are induced by hydrogenabsorption in the sintered alloy, leading to deterioration of mechanicalstrength, which is not preferred. Therefore, in the case where the Curietemperature of a sintered RE-containing alloy is controlled throughhydrogen absorption, the RE-containing alloy preferably contains α-Fe inthe alloy microstructure in an amount of at least 5 mass % so as tomaintain mechanical strength of the sintered alloy. In other words, inorder to satisfy both pulverization efficiency and mechanical strengthof the sintered alloy, the RE-containing alloy preferably contains α-Fein an amount of 5-15 mass % inclusive.

The RE-containing alloy may further contain, other than the NaZn₁₃structure and α-Fe, a second phase in the microstructure. The secondphase is provided for exerting an effect for enhancing thermalconversion efficiency by increasing the width of a peak attributed toentropy change.

According to the method for producing a sintered RE-containing alloy ofthe present invention, a plurality of long-term heat treatments, whichhave conventionally been performed, can be omitted, thereby reducing theoxygen concentration. Specifically, a sintered alloy having an oxygenconcentration of 5,000 ppm or less can be produced.

The sintered RE-containing alloy produced according to the presentinvention is corroded when exposed to the air or to a wet atmosphere.Therefore, in accordance with needs, progress of corrosion can beprevented by coating with resin or metal. Through coating, the oxygenconcentration and the nitrogen concentration in the sintered alloy canbe suppressed to 5,000 ppm or less.

According to the present invention, the oxygen concentration of alloypowder, sintered alloy, and compacts can be suppressed to a low level.Thus, even when rare earth metals serving as starting materials have apurity of about 98 mass %, satisfactory characteristics can be attained.

When the RE-containing alloy powder of the present invention is employedas a magnetic refrigerant, absorption of hydrogen into the RE-containingalloy before pulverization provides the following two points: magneticphase transfer temperature can be satisfactorily controlled, and ease ofpulverization can be enhanced.

The sintered RE-containing alloy produced according to the presentinvention can be employed as a magnetostrictive material for producing amagnetostrictive device. Specifically, the device is produced by windinga coil around the sintered alloy product and works by changing amagnetic field to cause dimensional changes of the sintered alloyproduct. When the alloy is employed as a magnetic refrigerant, the alloyis charged, in the form of a plate, porous sintered product, or powder,into a tube through which a cooling medium is passed.

EXAMPLES

The present invention will next be described by way of an Example and aComparative Example.

Example 1

Metallic La (serving as a source of rare earth metal (R)), electrolyticiron (serving as a source of transition metal (T)), and metallic Si(serving as a source of element (A)) were weighed and mixed such that analloy composition of La(Fe_(0.88)Si_(0.12))₁₃ was attained. The mixturewas melted in an Ar atmosphere at 0.1 MPa, by means of heating to 1,600°C.

Subsequently, by means of a strip casting apparatus shown in FIG. 1, themolten metal was poured, onto a copper roller which was cooled withwater and rotating at a rotating speed of 0.882 m/s, at a pour rate of150 g/s and a width of 85 mm, to thereby rapidly quench the moltenmetal, whereby alloy strips having a thickness of 0.28 mm were produced.The cooling rate as measured within a range of 1,600 to 900° C. wasfound to be approximately 1×10³° C./sec.

FIGS. 10 and 11 are back-scattered electron images of a cross-section ofa sample of the produced alloy strips. The image of FIG. 11 is anenlarged image of FIG. 10.

As shown in the FIGs., the alloy strip sample was found to have a minutemicrostructure in which an R-rich phase (white portion) had a size of 5μm or less and an R-poor phase (gray portion) had a size of 10 μm orless.

The thus-produced alloy strips were heated at 1,100° C. for three hoursin a vacuum atmosphere (≦5 Pa), followed by pulverization. The formedpowder was analyzed through powder X-ray diffractometry. Throughanalysis, a peak attributed to NaZn₁₃ structure (containing rare earthmetals) was observed, confirming that an NaZn₁₃ phase was formed.Through observation of a back-scattered electron image, the NaZn₁₃ phasecontent was found to be 90 vol. % or higher.

According to the present invention, a starting alloy in which an R-richphase and an R-poor phase are uniformly dispersed at a phase spacing of0.01 to 100 μm can be produced. From the starting alloy, an NaZn₁₃-typeRE-containing alloy having an NaZn₁₃ phase content of at least 90 vol. %can be produced through heat treatment for a time as short as one minuteto 200 hours.

Comparative Example 1

A molten metal was prepared in a manner similar to that of the Example,and an alloy was produced through the book mold method. Specifically,the molten metal was poured into a mold in which copper plates having athickness of 20 mm were juxtaposed at intervals of 30 mm, and allowed tostand for three hours for cooling to 50° C.

FIG. 12 is a back-scattered image of a cross-section of the producedalloy sample.

As is clear from comparison of FIGS. 10 and 12, the alloy of theComparative Example 1 produced through the book mold method has aconsiderably coarse alloy microstructure as compared with that of thealloy of Example 1 produced through rapid quenching. In addition, thealloy of the Comparative Example includes three or more phases; i.e., anR-rich phase, an R-poor phase, and one or more other phases, with eachphase having a size of 100 μm or more.

The alloy was heated at 1,100° C. for three hours, followed bypulverization. The formed powder was analyzed through powder X-raydiffractometry. Through analysis, in addition to a peak attributed toNaZn₁₃ structure, peaks attributed to α-Fe and an undesired phase wereobserved. Through observation of a back-scattered electron image of thealloy for analysis of metallographic microstructure and formed phases,the alloy was found to be formed of multiphases, and the undesired phasewas found to remain after heat treatment of 1,100° C. for 100 hours. Theresults indicate that removing the undesired phase present in the alloyrequires a long-period heat treatment.

Example 2

Metallic La (purity: ≧98 mass %) (6.5 at. %), pure iron (purity: ≧99mass %) (83.2 at. %), and metallic Si (purity: ≧99.99 mass %) (10.3 at.%) were melted under Ar, and the molten alloy was cast through stripcasting, to thereby produce alloy strips having a thickness of 0.25 mm.Subsequently, the thus-cast La—Fe—Si alloy was heated in a vacuum at1,100° C. for 12 hours.

The thus-produced La—Fe—Si alloy was pulverized by means of a disk mill,and the formed powder was classified by use of a sieve (100 μm). Thepulverization efficiency was found to be 1.2 kg/h. Subsequently, thethus-produced powder was compacted in a nitrogen atmosphere at acompacting pressure of 1.0 t/cm². The compact was sintered in a vacuumat 1,280° C. for three hours, followed by heating at 1,100° C. for 12hours, to thereby produce sintered La(Fe_(0.89)Si_(0.11))₁₃ alloy.

X-ray diffractometry of the sintered alloy shows that the relative peakintensity of La(Fe_(0.89)Si_(0.11))₁₃ alloy, having NaZn₁₃-type crystalstructure, is 93 mass %. Regarding a phase other thanLa(Fe_(0.89)Si_(0.11))₁₃ alloy, a peak attributed to α-Fe (7 mass %) wasobserved.

When the sintered alloy was maintained in hydrogen at atmosphericpressure at 270° C. for one hour, an increase (0.9%) in lattice constantwas identified through powder X-ray diffractometry, indicating thathydrogen was absorbed in the alloy. Cracking caused by an increase involume was not observed.

The oxygen concentration and the nitrogen concentration of the sinteredalloy were determined to be 2,250 ppm and 80 ppm, respectively.

Comparative Example 2

Metallic La (purity: ≧98 mass %) (6.5 at. %), pure iron (purity: ≧99mass %) (83.2 at. %), and metallic Si (purity: ≧99.99 mass %) (10.3 at.%) were melted under Ar, and the thus-produced La—Fe—Si alloy was castby pouring into a mold in which copper plates were juxtaposed atintervals of 30 mm. The cooling rate at the casting, as measured withina range of the temperature of the molten alloy to 900° C., was less than100° C./second.

The thus-obtained alloy was heated at 1,100° C. for 12 hours. The heatedalloy was found to contain α-Fe in an amount of 46 mass % andLa(Fe_(0.89)Si_(0.11))₁₃, having an NaZn₁₃ structure, in an amount of 54mass %. The thus-produced alloy was pulverized by means of a disk mill,and the formed powder was classified by use of a sieve (100 μm). Sincethe alloy has an α-Fe content in excess of 15 mass %, the pulverizationefficiency did not reach 0.2 kg/h. Subsequently, the thus-producedpowder was compacted in a nitrogen atmosphere at compacting pressure of1.0 t/cm². The compact was sintered in vacuum at 1,280° C. for threehours, followed by maintaining at 1,100° C. for 12 hours, to therebyproduce sintered La(Fe_(0.89)Si_(0.11) )₁₃ .

When the sintered alloy was maintained in hydrogen at atmosphericpressure at 270° C. for one hour, an increase (0.9%) in lattice constantwas identified through powder X-ray diffractometry, indicating thathydrogen was absorbed in the alloy. Cracking of the sintered alloycaused by increase in volume was not observed.

The oxygen concentration and the nitrogen concentration of the sinteredproduct were determined to be 6,200 ppm and 130 ppm, respectively.

The percent volume increase in a magnetic field of a device producedfrom the alloy was found to be 40% or less based on that of a deviceproduced from the alloy of the Example.

Comparative Example 3

Metallic La (purity: ≧98 mass %) (7.0 at. %), pure iron (purity: ≧99mass %) (82.7 at. %), and metallic Si (purity: ≧99.99 mass %) (10.3 at.%) were melted under Ar, and the molten alloy was cast through stripcasting, to thereby produce alloy strips having a thickness of 0.25 mm.Subsequently, the thus-cast La—Fe—Si alloy was heated in a vacuum at1,100° C. for 12 hours. The thus-produced alloy was pulverized by meansof a disk mill, and the formed powder was classified by use of a sieve(100 μm). The pulverization efficiency was found to be 1.3 kg/h.Subsequently, the thus-produced powder was compacted in a nitrogenatmosphere at compacting pressure of 1.0 t/cm². The compact was sinteredin vacuum at 1,280° C. for three hours, followed by maintaining at1,100° C. for 12 hours, to thereby produce sinteredLa(Fe_(0.89)Si_(0.11))₁₃.

The sintered product was found to have an La(Fe_(0.89)Si_(0.11))₁₃content, as measured from the relative peak intensity ofLa(Fe_(0.89)Si_(0.11))₁₃, of 99 mass %. Regarding a phase other thanLa(Fe_(0.89)Si_(0.11))₁₃, 1 mass % of α-Fe was determined from the peakintensity of α-Fe.

When the sintered product was maintained in hydrogen at atmosphericpressure at 270° C. for one hour, an increase (0.9%) in lattice constantwas identified through powder X-ray diffractometry, indicating thathydrogen was absorbed in the product. The sintered product of the solidform was cracked through absorption of hydrogen, to thereby form itspowder products.

INDUSTRIAL APPLICABILITY

The RE-containing alloy according to the present invention can beutilized to produce a magnetstrictive device and a magnetic refrigerant,and thus has industrial applicability.

1. A method for producing an RE-containing alloy represented by formulaR(T_(1−x)A_(x))_(13−y) (wherein R represents at least one speciesselected from among La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Tm, Yb, Gd, andLu; T represents at least one species selected from among Fe, Co, Ni,Mn, Pt, and Pd; and A represents at least one species selected fromamong Al, As, Si, Ga, Ge, Mn, Sn, and Sb (0.05≦x≦0.2; and −1≦y≦1))comprising a melting step of melting alloy raw materials at 1,200 to1,800° C.; and a solidification step of rapidly quenching the moltenmetal produced through the above step, to thereby form the firstRE-containing alloy, wherein the solidification step is performed at acooling rate of 10² to 10⁴° C./second, as measured at least within arange of the temperature of the molten metal to 900° C.
 2. The methodfor producing an RE-containing alloy according to claim 1, wherein, inthe melting step, the alloy raw material is melted in an inert gasatmosphere at 0.1 to 0.2 MPa.
 3. A method for producing the firstRE-containing alloy according to claim 1, wherein in the solidificationstep, the molten metal is rapid-quenched through any of strip casting,new centrifugal casting, and centrifugal casting.
 4. A method forproducing the RE-containing alloy according to claim 3, wherein themolten metal is rapidly quenched through strip casting in thesolidification step, to obtain strips having a thickness of 0.1 to 2.0mm.
 5. A method for producing an RE-containing alloy comprising amelting step and a solidification step for producing the RE-containingalloy according to claim 1, and a heat treatment step of heating at 900to 1,200° C. the RE-containing alloy that is produced through thesolidification step, to thereby form an NaZn₁₃ phase.
 6. The method forproducing an RE-containing alloy according to claim 5, wherein theNaZn₁₃ phase is formed through the heat treatment step, which isperformed for a period of from one minute to 200 hours.
 7. The methodfor producing the RE-containing alloy according to claim 6, wherein theheat treatment is performed at a temperature of 1080° C. to 1200° C. andfor a period of from 3 to 42 hours.
 8. An RE-containing alloy which isobtainable through the method of any one of claims 1 to
 4. 9. AnRE-containing alloy, which is represented by the formulaR(T_(1−x)A_(x))_(13−y) (wherein R represents at least one speciesselected from among La, Ce, Pr, Nd, Sm, Eu, Th, Dy, Ho, Tm, Yb, Gd, andLu; T represents at least one species selected from among Fe, Co, Ni,Mn, Pt, and Pd; and A represents at least one species selected fromamong Al, As, Si, Ga, Ge, Mn, Sn, and Sb (0.05≦x≦0.2; and −1≦y≦1)), andwhich comprises an R-rich phase, having a relatively high rare earthmetal (R) content, and an R-poor phase, having a relatively low rareearth metal (R) content, wherein the R-rich phase and the R-poor phaseare dispersed at a phase spacing of 0.01 to 100 μm.
 10. An RE-containingalloy, which is represented by the formula R(T_(1−x)A_(x))_(13−y)(wherein R represents at least one species selected from among La, Ce,Pr, Nd, Sm, Eu, Tb, Dy, Ho, Tm, Yb, Gd, and Lu; T represents at leastone species selected from among Fe, Co, Ni, Mn, Pt, and Pd; and Arepresents at least one species selected from among Al, As, Si, Ga, Ge,Mn, Sn, and Sb (0.05≦x≦0.2; and −1≦y≦1)), wherein the alloy has anNaZn₁₃ phase content of at least 90 vol. %.
 11. A magnetostrictivedevice provided from the RE-containing alloy according to claim
 10. 12.Amagnetic refrigerant provided from the RE-containing alloy according toclaim
 10. 13. An RE-containing alloy, which is represented by acompositional formula of R_(r)T_(t)A_(a) (wherein R represents at leastone rare earth element selected from among La, Ce, Pr, Nd, Sm, Eu, Tb,Dy, Ho, Tm, Yb, Gd, and Lu; T collectively represents transition metalelements containing at least Fe atoms, a portion of the Fe atoms beingoptionally substituted by at least one species selected from among Co,Ni, Mn, Pt, and Pd; A represents at least one element selected fromamong Al, As, Si, Ga, Ge, Mn, Sn, and Sb; and r, t, and a have thefollowing relationships: 5.0 at. %≦r≦6.8 at. %, 73.8 at. %≦t≦88.7 at. %,and 4.6 at. %≦a≦19.4 at. %) and having an alloy microstructurecontaining an NaZn₁₃-type crystal structure in an amount of at least 85mass % and α-Fe in an amount of 5-15 mass % inclusive.
 14. A method forproducing an RE-containing alloy powder, comprising pulverizing, bymechanical means, the RE-containing alloy according to claim 13 to apowder having a mean particle size of 0.1 μm to 1.0 mm.
 15. AnRE-containing alloy powder comprising an RE-containing alloy accordingto claim 13, which has a mean particle size of 0.1 μm to 1.0 mm.
 16. Amagnetic refrigerant comprising the sintered RE-containing alloy powderaccording to claim 15, wherein the Curie temperature of the magneticrefrigerant has been controlled through absorption of hydrogen in thesintered RE-containing alloy.
 17. A method for producing a sinteredRE-containing alloy, which comprises compacting an RE-containing alloypowder produced through a method for producing an RE-containing alloypowder according to claim 14, and sintering the compact.
 18. The methodfor producing a sintered RE-containing alloy according to claim 17,wherein the sintering is performed at 1,200° C. to 1,400° C.
 19. Themethod for producing a sintered RE-containing alloy according to claim17 or 18, wherein, after completion of sintering the RE-containing alloypowder, the sintered alloy is maintained in a hydrogen atmosphere at200° C. to 300° C., to thereby absorb hydrogen into the sintered alloy.20. A sintered RE-containing alloy, which is formed by compacting theRE-containing alloy powder according to claim 15, and sintering thecompact.
 21. A magnetostrictive material comprising the sinteredRE-containing alloy according to claim 20, wherein the Curie temperatureof the magnetostrictive material has been controlled through absorptionof hydrogen into the sintered RE-containing alloy.
 22. A magneticrefrigerant comprising the sintered RE-containing alloy as recited inclaim 20, wherein the Curie temperature of the magnetic refrigerant hasbeen controlled through absorption of hydrogen into the sinteredRE-containing alloy.